1. Field of the Invention
The present invention concerns an industrial type vented cell aqueous electrolyte secondary storage cell requiring no maintenance.
2. Description of the Prior Art
A storage battery is of the xe2x80x9cindustrialxe2x80x9d type if it has a high capacity, greater than 5 Ah and generally in the range 10 Ah to 200 Ah. A storage battery is of the xe2x80x9copenxe2x80x9d type if it operates at a relative pressure of not more than one bar (pressure difference relative to atmospheric pressure), the maximal operating pressure being fixed by the pressure at which a relief valve opens. An industrial type vented cell storage battery usually has a prism-shape plastics material container which contains plane electrodes. These aqueous alkaline electrolyte storage batteries are usually based on nickel-cadmium (Nixe2x80x94Cd) and nickel-metal hydride (Nixe2x80x94MH).
An industrial type vented cell storage battery includes an electrode package comprising a plurality of pairs. Each pair is made up of a positive electrode, a negative electrode and a separator between the positive and negative electrodes that is virtually impermeable to gases. The electrode package is immersed in a free electrolyte the quantity of which is much greater than that strictly required.
The operation of aqueous electrolyte storage batteries causes gases to be released at the end of charging, namely oxygen at the positive electrode and hydrogen at the negative electrode. A vented cell storage battery operates at a low pressure. The gases produced at the end of charging escape via the relief valve, which leads to consumption of the water of the electrolyte. The loss of water is estimated at approximately 0.3 cm3 per ampere-hour. The storage battery therefore requires regular maintenance, in other words water must be added periodically. The maintenance frequency depends on the conditions of use of the storage battery in the application concerned, in particular the charged capacity.
To limit the frequency of topping up the electrolyte level after periods of operation, semi-flooded cell storage batteries have been developed in which the electrode package is immersed only partly in the electrolyte, which allows oxygen generated at the positive electrode access to the non-immersed part of the negative electrode, and so recombination occurs here (EP-0 070 646).
Under the above conditions recombination is never complete and these storage batteries still require maintenance. Also, the non-immersed part of the negative electrodes is exposed to an oxygen atmosphere which can degrade it.
To eliminate all maintenance sealed cell industrial type storage batteries derived from those previously described have been proposed (EP-0 666 608). A sealed cell storage battery operates at a relative pressure that can be higher than 1 bar. In a sealed cell storage battery the electrode package includes a separator between the negative and positive electrodes that is highly permeable to gases, allowing oxygen generated at the positive electrode access to the negative electrode, and so recombination occurs there. A sealed cell storage battery further includes an oxygen recombination device. The oxygen formed at the positive electrode at the end of charging leads to an increase in the internal pressure of the storage battery which depends on the charging current. Afterwards permanent conditions are established in which all of the oxygen produced at the positive electrode is reduced (or xe2x80x9crecombinedxe2x80x9d). The electrolyte is introduced in limited quantities corresponding to the pore volume available in the components (electrodes, separator, recombination device).
The generation of gases can expel electrolyte from the pores in the electrode. This phenomenon shortens the service life of the battery by precipitating drying out of the electrode package. Document FR-1 012 395 proposes to provide a low-capacity sealed cell Nixe2x80x94Cd storage battery with a volume in which the electrolyte expelled in this way can collect. The separators dip into the electrolyte and return the electrolyte to the electrode package.
Nixe2x80x94MH storage batteries have a negative electrode in which the electrochemically active material is a hydridable metal alloy. Operation of the electrode in an aqueous medium means that corrosion of the hydridable alloy is inevitable. This reaction leads to additional consumption of water.
Where the electrolyte is introduced in limited quantities into the pores of the components, corrosion leads to premature drying out of the storage battery: its capacity falls and its service life is shortened. Document JP-8-96833 proposes to add in the lower part of an Nixe2x80x94MH storage battery a chamber containing the electrolyte and separated from the electrode package by a sealed wall. An extension of the separator at the bottom of the electrode package passes through this wall and enters the chamber.
Although this increases the service life, the capacities of sealed cell storage batteries are lower than those of vented cell storage batteries. This is because, during the first cycles, the electrodes do not have enough electrolyte to reach their maximum efficiency. Subsequent addition of electrolyte will not have any effect on efficiency.
The aim of the present invention is to propose an industrial type vented cell storage battery that does not require any maintenance and whose energy per unit mass, energy per unit volume and the service life are increased over those of an industrial type sealed cell storage battery.
The present invention consists in an industrial type vented cell storage battery comprising:
an electrode package including at least one positive electrode containing nickel hydroxide, one negative electrode, and one hydrophilic and gas-permeable separator an extension of which beyond the electrode package is in contact with electrolyte contained in a space between the base of the electrode package and the bottom of the container adapted to contain at least part of an excess quantity of alkaline electrolyte, and
an oxygen recombination device.
Industrial type vented cell storage batteries usually have a prism-shape plastics material (polypropylene) container incapable of withstanding a high pressure (greater than 2 bars relative). In the storage battery of the present invention the separator is permeable to gases to allow oxygen generated at the positive electrode access to the negative electrode. The presence of a recombination device is nevertheless essential because it significantly increases the rate of recombination and enables equilibrium to be obtained at a moderate pressure, even when high charging currents are used. The recombination device includes one or more recombination sites connected to the negative polarity at which the reaction occurs. The device preferably includes a spacer in order to provide a constant volume for gas circulation. The device can include a spacer associated with an auxiliary electrode. A recombination system of the above kind is described in document EP-A-0 666 608 published Sept. 8, 1995, for example.
An industrial type vented cell storage battery is flooded with electrolyte to obtain a high electrode efficiency, higher than that observed in a sealed cell storage battery containing a quantity of electrolyte limited to the volume of the pores of the components. The storage battery of the present invention is flooded with electrolyte (i.e. it contains electrolyte in excess quantity) and includes a space capable of containing at least some of this excess electrolyte. The separator in contact with this electrolyte irrigates the electrode package by capillary action.
This space is between the base of the electrode package and the bottom of the container so that an extension of the separator beyond the bottom of the electrode package dips into the electrolyte contained in it. It can be obtained by raising the electrode package relative to the bottom of the storage battery. This arrangement has the advantage that some of the excess electrolyte is under the electrode package, instead of covering it. Access of the gas to the recombination device will be possible sooner and the storage battery will achieve quasi-sealed operation (without the relief valve opening) faster. Electrolyte losses from the storage battery of the invention will therefore be reduced compared to a prior art storage battery.
During cycling there are three operating phases of the storage battery of the invention. During the first cycles the electrodes obtain their maximum efficiency because the separator supplies them with as much electrolyte as necessary, taken from the excess quantity. The recombination device has not yet reached its full efficiency and the internal pressure can reach values greater than that at which the relief valve opens. There is then a low consumption of electrolyte in each cycle because the relief valve allows the gases to escape. This first phase, which is of very short duration, is not always present. It is succeeded by a second phase in which recombination is effected efficiently and in which the pressure tends to stabilize. Finally, after a few charge/discharge cycles, the internal pressure remains below the pressure at which the relief valve opens. The recombination rate is virtually 100% which means a behavior close to that of a sealed cell storage battery.
After prolonged cycling, the electrodes can dry out, which is a frequent cause of faults in sealed cell storage batteries. In the storage battery of the invention the separator in contact with the excess electrolyte irrigates the electrode package by capillary action during cycling. The service life of the open cell storage battery of the invention is very much longer than that of a sealed cell storage battery.
In the storage battery of the present invention the quantity of electrolyte introduced corresponds to the volume of electrolyte absorbed by the electrode package (electrodes and separator) and the recombination device, constituting a reference quantity, increased by an excess quantity. The total quantity of electrolyte is preferably at least equal to 120% of the quantity of electrolyte contained in the total pore volume of the electrodes, the separator and the device. The excess quantity therefore represents at least 20% of the reference quantity. For example, storage batteries have been constructed containing a quantity of electrolyte equal to 145% and 126% of the quantity of electrolyte contained in the total pore volume.
The total capacity of the negative electrodes is usually greater than that of the positive electrodes. This negative capacity excess is used in a sealed cell storage battery to prevent the release of hydrogen that cannot recombine. In an Nixe2x80x94MH storage battery it compensates the drop in negative capacity due to corrosion of the alloy.
In a vented cell storage battery the negative electrodes have an overcapacity relative to the capacity of the positive electrodes which is less than in a sealed cell storage battery. The electrodes of the two polarities are overcharged in each cycle and the negative capacity excess merely compensates the effect of corrosion (reduced quantity of active material). In accordance with the present invention, the negative electrodes preferably have a total capacity in the range 100% to 150% of the total capacity of the positive electrodes.
In one variant, the positive electrodes are of the sintered type and the negative electrodes are of the paste type on a conductive support chosen from a two-dimensional support, such as a solid or perforated tape, an expanded metal, a grid or a woven material, and a three-dimensional support, such as a foam or felt support.
In another variant, the positive electrodes are of the paste type on a three-dimensional conductive support and the negative electrodes are of the paste type on a conductive support chosen from a two-dimensional support and a three-dimensional support.
The storage battery of the invention can include negative electrodes containing a hydridable metal alloy as the electrochemically active material.
The maintenance-free storage battery of the invention is particularly suitable for use in aeronautical or railroad applications and for propulsion of electric vehicles.